At the present time, telecommunication systems are largely based on fiber optic cables. For example, optical networks based on fiber optic cables are currently utilized to transport Internet traffic and traditional telephony information. In such applications, it is frequently necessary to provide an optical signal over significant distances (e.g., hundreds of kilometers). As optical signals travel through the optical fibers, a portion of their power is transferred to the fiber, scattered, or otherwise lost. Over appreciable distances, the optical signals become significantly attenuated. To address the attenuation, optical signals are amplified. Typical optical amplifiers include rare earth doped amplifiers (e.g., Erbium-doped fiber amplifiers).
Also, Raman amplifiers may be utilized. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is conserved as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
The probability that a Raman scattering event will occur is dependent on the intensity of the light as well as the wavelength separation between the two photons. The interaction between two optical waves due to SRS is governed by the following set of coupled equations:                     ⅆ                  I          P                            ⅆ        z              =                            -                                    λ              S                                      λ              P                                      ⁢                  g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          P                ⁢                  I          P                                        ⅆ                  I          S                            ⅆ        z              =                            g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          S                ⁢                  I          S                    
where Is is the intensity of the signal light (longer wavelength), Ip is the intensity of the pump light (shorter wavelength), gR is the Raman gain coefficient, xcexs is the signal wavelength, xcexs is the pump wavelength, and xcex1s and xcex1p are the fiber attenuation coefficients at the signal and pump wavelengths respectively. The Raman gain coefficient, gR, is dependent on the wavelength difference (xcexsxe2x88x92xcexp) as is well known in the art.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have come into existence only over the past few years. These advances have renewed interest in Raman amplifiers.
A key performance parameter of Raman amplifiers is the gain flatness of the amplifier. Gain flatness can be quantified by measuring the gain ripple (variation in gain experience by the optical channels) across the amplification band. To compensate for non-zero gain ripple, a gain flattening filter may be applied to the optical signal to equalize the gain between channels. However, this is a non-optimal solution, since this approach adds loss and therefore decreases the signal-to-noise (SNR) ratio of the system.
Additionally, a single wavelength pump source generates the gain spectrum depicted in FIG. 1. The gain spectrum is not exceptionally broad and it does not possess low gain ripple. However, for telecommunications systems, it is desirable to achieve low gain ripple over both the C Band (wavelengths from 1530 to 1565 nm) and L band (wavelengths from 1570 to 1610 nm). Additionally, it is anticipated that telecommunication service providers may begin to utilize the S band (wavelengths from 1480 to 1525 nm) and the XL band (wavelengths from 1615 to 1660 nm). However, a single wavelength pump source is not capable of generating flat Raman gain across an entire band.
Accordingly, a plurality of lasers, each operating out a distinct wavelength have been utilized to provide optical gain across a broad amount of spectrum. An example, which uses wavelength division multiplexed lasers, is discussed in Pump Interactions in 100-nm Bandwidth Raman Amplifier, H. Kidorf et al., IEEE Photonics Technology Letters, Vol. 11, No. 5, May 1999. Other systems have utilized individually packaged fiber Bragg grating stabilized pump lasers. The individually packaged lasers are placed in, for example, 14 pin butterfly packages. The output beams from the individual devices are either polarization division multiplexed or wavelength division multiplexed into a single beam. To the extent that these systems use more beams, the systems are able to generate a broad and reasonably flat gain spectrum. However, the systems become quite cumbersome and costly when the number of butterfly packages exceeds a relatively small number. Accordingly, the gain flatness that can be achieved cost-effectively is severely limited.
The present invention is directed to a system and method for providing a spectrally tailored Raman pump. The system and method employ an incoherently beam combined laser configuration to combine output beams from a plurality of emitters with each of the emitters providing a different output wavelength. Embodiments of the present invention provide emitter devices with electrodes adapted to allow addressability of various emitters. In some-embodiments, each emitter is individually addressable, thereby allowing the output power of each emitter to be controlled by a drive current. In another embodiment, blocks of emitters are coupled to a single current source. Each emitter of a block is operated at a common current level. In certain embodiments, blocks of emitters are driven at current levels significantly greater than the threshold current for the emitters to optimize the electrical efficiency of the device. Moreover, certain embodiments vary emitter spacing to increase linear power density and/or to allocate additional power to the blue (shorter wavelength) end of the Raman pump. By providing spectral tailoring, embodiments of the present invention are capable of providing flat Raman gain over a broad spectrum.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.